Because of the need to increase the power and battery life of cellular phones, while also reducing the size of the phones and their components, there is currently significant research being conducted on the design of more efficient linear power amplifiers that may be used in handset applications. The use of linear power amplifier technology over non-linear amplifiers is particularly important for efficient handset applications using transmission systems that incorporate non-constant envelope modulation schemes, such as those found in code division multiple access modulation schemes like W-CDMA, IS-95 and CDMA2000.
Power amplifiers for these applications are typically fabricated on materials such as Gallium Arsenide (GaAs), and its variants, which have been found to be advantageous for use in power amplifiers for cellular applications, due to the efficiency gains that may be achieved using these semiconductors.
There also has been significant efforts placed in implementing more efficient dual-, tri-, and quad-band power amplifiers. However, the power amplifier for one transmission band in these solutions has been implemented as a stand-alone device, due to significantly different requirements between the transmission standards. This has required the use of two or more amplifier dies, which increases the size of the amplifier and significantly increases the cost, reducing their desirability in cellular applications. As an example, in solutions for GSM applications, the W-CDMA power amplifier is a separate, stand-alone unit because of its different RF requirements.
It would be advantageous to further enhance the efficiency and power output of linear amplifiers used in such applications, and for providing a power amplifier optimized for one transmission band, such as W-CDMA, to be more efficiently operable at one or more other bands, such as at PCS frequencies and output power levels.